In an inactive air conditioning, heat pump, or refrigeration system, refrigerant tends to condense and collect at cool and/or low locations in the system. For the range of indoor and outdoor temperatures encountered in many systems during the off-portions of their cycles, the compressor is often the coolest part of the system for some period of time. As a result, considerable liquid refrigerant may collect in both suction-side and discharge-side portions of the compressor, possibly degrading compressor reliability in several ways.
Liquid refrigerant that collects in the compressor oil sump dilutes the oil, reducing its ability to lubricate compressor bearings and other moving parts when the compressor is started. Liquid that condenses on the suction side of a compressor may be drawn into the compression mechanism at start-up and wash away lubricating oil films normally present on moving parts. Liquid that condenses on the suction side may also be delivered directly or indirectly into the compressor oil sump at start-up, thereby diluting oil with the possible consequences described above.
Because of the affinity between refrigerants and many of the lubricants used therewith, refrigerant may also migrate to, and dissolve into, the oil over time even when the compressor is not any cooler than other portions of the system, thereby contributing to oil dilution and attendant loss of lubricating ability. This phenomenon makes it very desirable to maintain a minimum oil charge amount-to-refrigerant charge amount ratio that will help ensure that the oil will have adequate lubricating quality even in the most diluted state. For the start-up transients of many typical systems using presently available refrigerants and oils, experimentation and field experience have resulted in an approximate guideline that the oil charge amount should be at least about 30 to 40% of the refrigerant charge amount.
To mitigate these problems a number of design approaches may be employed, one of which is a refrigerant accumulator. An accumulator is commonly used to collect liquid refrigerant on the suction-side of the compressor and then meter its flow rate to the compressor compression mechanism and/or oil sump at start-up. A typical accumulator consists of an inlet pipe, a volume for storing liquid, a suction feed pipe that provides a passage for refrigerant gas to enter the compression mechanism or oil sump, and a circuitous flow path. The circuitous flow path is interposed between inlet pipe and inlet of suction feed pipe to divert liquid refrigerant, entering via the inlet pipe, away from the inlet of the suction feed pipe and into the storage volume, aided by the action of gravity. Often the circuitous flow path is created using a baffle in which holes or passages that allow the passage of refrigerant are vertically misaligned relative to the inlet of the suction feed pipe. Other arrangements that do not use a baffle are also possible, such as vertical misalignment of inlet pipe and suction feed pipe inlet, for example.
At start-up, low pressure caused by the action of the compression mechanism tends to vaporize any liquid refrigerant residing in the liquid storage volume and draws the vapor towards the compression mechanism via the suction feed pipe. Converting the liquid refrigerant to vapor in this manner helps avoid the washing of oil films and/or dilution of sump oil that might otherwise occur if liquid refrigerant were allowed to enter the compression mechanism or sump directly.
In practice, a small amount of liquid still may still enter the compression mechanism and/or sump via one or more "bleed holes" or metering ports located in the suction feed pipe and connecting its passage with the storage volume. A metering port is necessary to prevent accumulation of oil that normally circulates through the system with refrigerant in small percentages. Upon reaching the accumulator inlet pipe via piping connecting the evaporator and accumulator, oil may be in any or all of several forms. Oil may arrive as a film on interior walls, pushed along by the flow of refrigerant vapor. Oil may also arrive in mixture with or solution with liquid refrigerant. Some oil may also be entrained in refrigerant vapor as a mist. The circuitous path used to divert liquid refrigerant into the storage volume is also effective in diverting oil into the storage volume. Additionally, when oil enters the accumulator as a mist entrained in refrigerant vapor, the circuitous path requires changes in flow velocity and direction and may enhance flow impingement with some of the accumulator's internal solid surfaces. These tend to cause some portion of any oil entrained in the flowing refrigerant vapor to become separated out, where it flows to and collects at the bottom of the accumulator as a consequence of gravity.
Without a least one metering hole, over time the accumulator storage volume might become filled with separated oil. As accumulator storage volumes are typically a considerable fraction of oil sump volumes, and may exceed oil sump volumes, the resulting reduction of oil level in compressor sump could very well result in eventual failure by loss of lubrication. To minimize the amount of residual oil that may be prevented from returning to the compressor sump, at least one metering port is conventionally located near the bottom of the accumulator storage volume. In instances where an immiscible or partially miscible oil is used, metering ports are located at several elevations along the suction feed pipe of the accumulator. In this way, oil or an oil-rich mixture of oil and refrigerant floating on top of liquid refrigerant can be fed into the suction feed pipe. For these cases, at least one metering port is still located near the bottom of the storage volume to minimize the amount of residual oil that may be trapped in the accumulator when no liquid refrigerant is present. The lowest practical location of a metering hole in conventional accumulators is determined by manufacturing process and assembly tolerances. Typically, a metering hole may be located such that, for the worst-case assembly, trapped residual liquid volume may be a fraction of a fluid ounce in a small accumulator to as much as about a fluid ounce (30 cc) in a large accumulator. In conventional accumulators, trapped residual liquid volumes are on the order of a few percent to, in extreme cases, perhaps as much as 10% of the accumulator liquid storage volume.
When the storage volume contains liquid refrigerant, the metering port allows liquid refrigerant to flow into the suction feed pipe as well as oil. Accumulators can still be effective as long as the metering port is correctly sized and the amount of liquid refrigerant entering the accumulator does not exceed its storage capacity. If the metering port is too large, then, during operation with liquid refrigerant entering the accumulator, the flow rate of liquid refrigerant metered into the suction feed pipe may exceed rates required for reliable operation. If the metering port is too small or the accumulator storage capacity is too small for the system refrigerant charge, then liquid refrigerant may overflow the inlet of the suction feed pipe. If this happens, the rate of liquid flow to the compression mechanism and/or oil sump could be substantially increased. As this liquid is predominantly liquid refrigerant, washing of oil films from moving parts and/or dilution of oil in the sump may result.
As mentioned earlier, another effective and desirable approach is to design the compressor oil sump to hold an amount of oil that is at least about 30 to 40% of the system refrigerant charge amount, and preferably greater. Trends in system design are making this increasingly difficult and costly, however. Increasingly, consumer preference is towards systems with longer refrigerant connection lines and multiple indoor units serviced by a single outdoor unit and compressor. Both of these trends may result in systems with increased refrigerant charge amounts.
Redesigning compressor shells to increase oil sump volume may be costly, particularly insofar as these increases may exceed the capabilities of existing capital equipment. Simply adding additional oil to compressors without changing the volumes of their sumps may also be undesirable. The resulting increase in oil level may cause increased oil circulation rates due to impingement by moving parts or entrainment in discharged gas. Increased oil circulation rate, in turn, could degrade performance of heat transfer surfaces, thereby causing a loss of system efficiency.
If additional oil storage could be provided in a cost-effective manner in a compressor component that refrigerant may enter and leave, the oil to refrigerant ratio of the system could be increased without increasing the storage capacity of the oil sump. Further, if this additional oil storage could be provided in the suction accumulator, then during those times when primarily liquid refrigerant would otherwise be delivered to the compressor, a mixture of liquid refrigerant and oil would be delivered, having much better lubricating ability than liquid refrigerant. After oil has left the accumulator in this manner, a subsequent period of normal operation would allow the liquid separating mechanisms inherent in suction accumulators to replenish the oil stored in the accumulator, thereby allowing the additional oil stored in the accumulator to perform its functions repeatedly during the life of the system.